This invention relates generally to high performance springs, and more particularly to leaf springs used in measuring apparatus for very light masses and forces.
Mechanical springs used in measuring apparatus for very light masses and forces, such as gem scales and load cells, have stringent sensitivity requirements. The spring must be highly sensitive to load, and must accurately duplicate displacements for various loads. With leaf-type springs, this requires a very thin leaf member for sensitivity and a flat, rigid spring footing to prevent peeling stresses between the foot bottom and the surface it is adhered to, and thereby to insure accurate repeatability in displacements.
Heretofore, rigidity in the foot of leaf springs has been provided through thickness. Either the thin spring member is sandwiched between two thicker members at the foot, or the thin spring member is machined from a thick section making up the spring foot. Either approach has drawbacks when applied to leaf springs used in gem scales or load cells, especially of the type disclosed in my copending patent application, Ser. No. 07/018,176, filed 02/24/87, which discloses an electronic balance for measuring very light masses and capable of correcting an error in the measured value due to off-center placement of the mass to be weighed. Essential to the operation of said balance are the deflection measurements accomplished using a plurality of capacitors formed between two rigid, closely-spaced plates with specific patterns on the plates forming the capacitors. The plates are held apart by three or more springs Spaced around the periphery of the plates and attached thereto.
Leaf springs used in measuring apparatus for very light masses have a very thin leaf member. For strength, flexibility, repeatability and sensitivity, the leaf spring member is generally metallic. Because such a member is highly sensitive to very light loads, i.e., to 0.01 carat, it is also very sensitive to heat. The thicker a metal is, the longer it will take to thermally neutralize itself when subject to temperature changes, whereas a thin metal will neutralize itself quickly to the same temperature as the surrounding ambient air. Since the characteristics of metal change with heat, an apparatus with a thicker leaf spring, i.e., the leaf spring member or the leaf spring footing, may take up to one hour to thermally neutralize itself after being subject to a temperature change.